1. Technical Field
The present invention relates in general to an improved system for reducing the wear of components in a well, and in particular to an improved system for remotely adjusting the centering of a drill string in a well to prevent damage to the wellhead.
2. Description of the Prior Art
In offshore wellhead equipment, there are instances in which an inner tubular member must be releasably locked into an outer tubular member within a well. For example, while drilling an offshore well with a jack-up drilling rig, a wellhead housing with a blowout preventer (BOP) is located on a string of casing that extends upward from the sea floor. The wellhead housing is located on a well deck below the rig floor. A riser extends upward from the wellhead housing to the rig floor. The drilling rig runs drill pipe down through the wellhead housing for drilling purposes. It is important to avoid damaging the bore of the wellhead housing and also the seal where it connects to the riser.
In the prior art, wear bushings are often deployed to prevent damage to the wellhead from the rotating drill pipe. Wear bushings are retained in the bore and installed remotely by lowering them through the riser. However, wear bushings are subject to a number of limitations and problems. For example, without some type of retention mechanism, a wear bushing can be dislodged by circulation of heavy solids or by tripping of the drill pipe through the wellhead during normal drilling operations. If the wear bushing is dislodged, it could become repositioned in the blowout preventer stack and cause damage to or failure of the blowout preventer to shut in the well during a pressure kick. Such a condition could subject the rig to a blowout, causing serious damage. Although there are various mechanisms for retaining wear bushings, such as shear pins, lock rings, and J-pins made of steel or other metallic alloys, users have experienced failure in activating or releasing these devices. It is difficult to recover the wear bushing if the locking mechanism fails to release.
Another problem with wear bushings is that they must be replaced occasionally during use, and then retrieved after drilling operations are complete. The time required to stop drilling, retrieve the wear bushing, and then replace it with a new one before recommencing operations is costly. Moreover, wear bushings are limited to a single size or internal diameter. Since the bore sizes of a single well may range from 7.5 inches to 18.75 inches, an unspent wear bushing must be replaced if the tooling required during operation is larger or smaller than the internal diameter of the wear bushing. Thus, an improved system for protecting wellhead assemblies is needed.
A drill bit guide is mounted above a wellhead in place of a wear bushing. The bit guide is capable of guiding strings and tools through the wellhead without damage to the wellhead or string while drilling. In one version of the bit guide, a pair of linear actuators radially extend and retract separate halves of the bit guide to conform to the size of the object located between them. In another version of the bit guide, a set of interlocking arms and wear bars are articulated to form a circular opening having a variable inner diameter. A drill string may be lowered through a fully open bit guide or landed on top of a fully closed bit guide. The bit guide also can be moved to more closely receive the drill string passing through it to prevent damage to the drill string and the wellhead.
The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the preferred embodiment of the present invention, taken in conjunction with the appended claims and the accompanying drawings.